The development of InGaAs photodetectors has received considerable attention in recent years for long-haul applications in the 1.3 to 1.6 .mu.m region of the spectrum. One type of structure that has been investigated is the interdigitated-metal-semiconductor-metal (IMSM) photodetector, which has a planar geometry and simple process that is attractive for monolithic integration with electronic circuits. InGaAs/InP IMSM photodetectors have been demonstrated with good performance and integrated with simple InP-based electronic circuits. The relative immaturity of InP-based electronics, however, has motivated photodetector research to investigate growing InGaAs on GaAs substrates, which offers the potential of exploiting manufacturing-based GaAs electronics technology. One issue that will determine the viability of this approach is the process compatibility of the photodetector and the electronic components.
Commercial GaAs IC technology, which is based on the metal-semiconductor field-effect transistor (MESFET), is typically fabricated by an ion implantation process that requires a high-temperature anneal step. Fabrication of an InGaAs/GaAs monolithic receiver will, therefore, require that the photodetector material be capable of withstanding the high temperature anneal without degradation of its optical and electrical properties.
InGaAs IMSM structures are generally fabricated with a thin cap layer of a large bandgap material, such as GaAs, AlGaAs or InAlAs to enhance the low Schottky barrier height (.about.0.3 eV) between the InGaAs and most metals. Two such structures were disclosed by Razeghi et al. in "Planar Monolithic integrated photoreceiver for 1.3-1.55 .mu.m wavelength applications using GaInAs-GaAs heteroepitaxies," Appl. Phys. Lett. Vol. 49, No. 4, p. 215 (1986) and by Garbiniski et al. in "Ga.sub.0.47 In.sub.0.53 As field-effect transistors with a lattice-mismatched reduced leakage current GaAs gate," Electron. Lett. Vol. 22, No. 5, p. 236 (1986).
Rogers et al. in "High Speed 1.3 .mu.m GaInAs Detectors Fabricated on GaAs Substrates," IEEE Electron Device Lett. 9, 515 (1988) demonstrated a four-layer IMSM photodetector grown by MBE with a 500.ANG. graded (In.sub.x Ga.sub.1-x As,x=0.4 to 0) GaAs cap layer. The first layer was a 0.5 .mu.m GaAs buffer layer followed by a 0.5 .mu.m of In.sub.0.25 Ga.sub.O.75 As layer. Next, a 1 .mu.m In.sub.0.4 Ga.sub.0.6 As absorbing layer was grown having a final GaAs thin (300.ANG.-500.ANG.) capping layer on top to enhance the Schottky barrier height of the IMSMs. When these layers are subjected to a high temperature annealing cycle, either in a furnace or in an RTA system as is required in ion-implanted GaAs MESFET processes, indium diffuses out from the absorbing (In.sub.0.4 Ga.sub.0.6 As) layer towards the surface. Indium diffusion disadvantageously shifts the absorption wavelength of the active layer (In.sub.0.4 Ga.sub.0.6 As) as well as degrades the Schottky barriers fabricated on this material. Thus, the annealing process results in a large increase in the photo-detector dark current compared to those fabricated on unannealed material.